As it turns out, the issue was caused by enabling IntelliTrace while running the worker role. Looking into it a bit further, IntelliTrace is causing this type of error to occur due to the dynamic IL it injects into the code which was is, in the case of
Json.NET, causing overall bad IL, this the exception.

Can you clarify please in which release this is fixed? I am using 4.0.2 and seeing the same issue with Intellitrace Enabled (in an Azure web role)
@ Kwal - did you need to deploy a private build of JSon from sources?